Originally Answered: Is it possible that there is life on Mars that is undiscovered?
If life started back then, as seems likely on Earth - then it may have started independently on Mars.
That was also a time when there was a lot of loose debris in the solar system. So if early life started on just one of the planets - so long as it evolved to the stage where it could withstand the vacuum and cosmic radiation to get transferred to another planet - it might well have been transferred between the two.
So, I think there is a reasonable chance that Mars had life in the past.
So, is it still there then?
Well first of all Mars probably has a hydrosphere - as you get far enough below the surface, ice melted, trapped by the layer of rocks above. You get life on Earth in similar conditions so it probably survives on Mars deep down so long as life ever managed to get there in the first place.
But it also may well survive on the surface.
The difficulty with the surface is that - though microbes on Earth can survive in dormancy for some millions of years, even tens and hundreds of millions of years - on Mars because of the cosmic radiation, it can only survive a few hundred thousand years at the outside in dormant state.
So - you need habitats that are there nearly all the time, not just at times when the Mars atmosphere is somewhat thicker due to changes in its orbit.
Either that or habitats some meters below the surface, protected from cosmic radiation - but most of Mars is completely frozen when you get as little as just 2 cms below the surface.
So, most scientists used to think that surface life was impossible, and present day below the surface, though possible, likely to be rare.
So - they thought that there is no life on Mars surface currently - though possibly in caves, geothermal hotspots and the hydrosphere.
That was almost a consensus (with one or two with dissenting views) until about 2008.
However, when they saw droplets on Phoenix's legs - that changed everything. They then began to model ways that water could exist on the surface of Mars - usually in presence of salt to reduce its boiling point.
And also they had observations of the Warm Seasonal Flows - there are many streaks and spots and lines on Mars that vary seasonally - but these particular ones only form on sun facing slopes when temperatures rise above 0C.
Because of that correlation, it is really hard to come up with any explanation for them that doesn't involve melting ice somewhere in the explanation. Then other streaks were also found in polar regions that also seemed to require ice to melt, and stay liquid. That can happen if the liquid is salty enough to depress the boiling point enough - and salts (in the form of sulfates, chlorates and perchlorates instead of the sulfides and chlorids more common on Earth) are common on Mars. Can also happen if the water is trapped, e.g. under a covering layer of ice with the solid state greenhouse effect.
Then when they analysed isotope measurements of Oxygen and Carbon in the atmosphere, again made by Phoenix, turned out that there must be some liquid, probably water, that exchanges oxygen atoms chemically with the CO2 in the atmosphere as an on going process - happening in geologically recent past.
Whether that is water present all the time, or episodes due to impacts or geothermal heat - either way it is promising for present day life on Mars.
Also DLR, the German aerospace company, found out that some lichens and cyanobacteria are able to metablize and do some photosynthesis in simulated Mars atmospheres using just the night time moisture (which goes up to 100% as the surface cools down) without any liquid as such.
Then, just earlier this year Nilton Remmo's team came up with a possible mechanism for water droplets to form rather readily when ice meets salts on Mars. As both salts and ice are common in the higher lattitudes, then this may be a widespread habitat, Nilton Remmo's "swimming pools for bacteria".
So putting all that together, it now seems possible, even verging on likely - that if there was life on Mars in the past or life ever got transferred from other planets or anywhere else in the solar system - that it is still there. Not just deep down but possibly on the surface too.
If so it will be pretty hard to spot though. Our rovers sent to Mars so far would not be able to spot life in the Atacama desert or the McMurdo dry valleys on Earth - except for the Viking labelled release experiment which was sensitive enough but produced controversial results because they didn't know about the unusual chemical composition of the Martian "soil" back then so didn't design it to take account of that.
Future rovers like ExoMars would be sensitive enough. But they still need to go to the right place on Mars. Here on the Earth in Antarctica then if you search for life, sometimes you find it, sometimes you don't, in apparently similar potential habitats. So it may be the same on Mars except - that most of Mars is nowhere near as hospitable as Antarctica. So it might be a long search to find it, even when we get to send rovers to the most likely places like the Warm Seasonal Flows. Or, who knows, maybe ExoMars will find present day life right away. No way to know for sure but I think most scientists would be surprised if it was as easy as that.
Best Places For Droplets, Films And Shallow Flows Of Liquid Water On Mars - Where Microbial Life May Flourish (plus other answers I did here on quora)
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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